Nicotinic receptors in the rat prefrontal cortex: increase in glutamate release and facilitation of mediodorsal thalamo-cortical transmission

Differential modulation of auditory thalamocortical and intracortical synaptic transmission by cholinergic agonist

To investigate synaptic mechanisms underlying information processing in auditory cortex, we examined cholinergic modulation of synaptic transmission in a novel slice preparation containing thalamocortical and intracortical inputs to mouse auditory cortex. Extracellular and intracellular recordings were made in cortical layer IV while alternately stimulating thalamocortical afferents (via medial geniculate or downstream subcortical stimulation) and intracortical afferents. Either subcortical or intracortical stimulation elicited a fast, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX)-sensitive, monosynaptic EPSP followed by long-duration, polysynaptic activity. The cholinergic agonist carbachol suppressed each of the synaptic potentials to different degrees. At low concentrations (5 microM) carbachol strongly reduced (>60%) the polysynaptic slow potentials for both pathways but did not affect the monosynaptic fast potentials. At higher doses (10-50 microM), carbachol also reduced the fast potentials, but reduced the intracortically-elicited fast potential significantly more than the thalamocortically-elicited fast potential, which at times was actually enhanced. Atropine (0.5 microM) blocked the effects of carbachol, indicating muscarinic receptor involvement. We conclude that muscarinic modulation can strongly suppress intracortical synaptic activity while exerting less suppression, or actually enhancing, thalamocortical inputs. Such differential actions imply that auditory information processing may favor sensory information relayed through the thalamus over ongoing cortical activity during periods of increased acetylcholine (ACh) release.

The brain alpha7 nicotinic receptor may be an important therapeutic target for the treatment of Alzheimer's disease: studies with DMXBA (GTS-21)

A large decrease in brain nicotinic receptor levels occurs in Alzheimer's disease, relative to muscarinic and other receptors. Neurons possessing high affinity nicotinic receptors seem particularly vulnerable. The low affinity nicotinic receptors which selectively bind alpha-bungarotoxin are not significantly affected. The major nicotinic receptor subtype which binds this toxin is a homo-oligomer composed of alpha7 subunits. Due to its exceptionally high calcium ion selectivity, this particular receptor can be considered as a ligand-gated calcium channel. Alpha7 receptors are found in regions of the brain which are important for cognition, including cerebral cortex and hippocampus. Hippocampal receptors are largely confined to GABAergic interneurons. Alpha7 receptors seem less likely than alpha4-beta2 receptors to be up-regulated in number and down-regulated in function as a result of chronic agonist exposure. A family of nicotinic agonists based upon the marine animal toxin anabaseine have been synthesized and investigated. One of these compounds, DMXBA [3-(2,4-dimethoxybenzylidene)-anabaseine; code name GTS-21] has displayed promising characteristics during phase I clinical tests. In the rat DMXBA is selectively agonistic upon alpha7 nicotinic receptors. In addition it is a moderately potent antagonist at alpha4-beta2 receptors. DMXBA enhances a variety of cognitive behaviors in mice, monkeys, rats and rabbits. It also displays neuroprotective activity upon cultured neuronal cells exposed to beta-amyloid or deprived of NGF. The compound is much less toxic than nicotine and does not affect autonomic and skeletal muscle systems at doses which enhance cognitive behavior. Phase I clinical tests indicate that large doses can be safely administered orally without adverse effects. Psychological tests on healthy young male subjects indicate a positive effect of DMXBA on some measures of cognition. While DMXBA is a much weaker partial agonist on human alpha7 receptors than upon rat alpha7 receptors, its 4-hydroxy metabolite has been shown to have excellent efficacy on both receptors. Thus, some of the physiological and behavioral effects of GTS-21 may be due to the actions of this primary metabolite.

Plasticity in the mediodorsal thalamo-prefrontal cortical transmission in behaving mice

We studied changes in thalamo-prefrontal cortical transmission in behaving mice following both low-frequency stimulation of the mediodorsal thalamus (MD) and during extinction of a conditioned fear response. Electrical stimulation of the MD induces a field potential in the medial prefrontal cortex (mPFC) characterized by two initial negative-positive complexes (N1-P1 and N2-P2) followed by two positive-negative complexes (P2-N3 and P3-N4). The N1-P1 and N2-P2 complexes were identified as resulting from orthodromic and antidromic prefrontal activation, respectively. Because the two complexes were not often easily dissociated, plasticity in the prefrontal synaptic transmission was considered to result from changes in N1-P2 amplitude. Low-frequency thalamic stimulation (1, 200 pulses at 2 Hz) produced either long-term (at least 32 min) depression or potentiation of the N1-P2 amplitude. Mice submitted to fear conditioning (tone-shock association), displayed on the first day of extinction (tone-alone presentations) a strong freezing behavior, which decreased progressively, but was still high the following day. Extinction of conditioned fear was accompanied the first day by a depression of prefrontal transmission, which was converted into potentiation the following day. Potentiation of prefrontal transmission lasted at least 24 h following the second day of the fear extinction procedure. In conclusion, low-frequency thalamic stimulation can produce, in behaving mice, either depression or potentiation of prefrontal synaptic transmission. Decrease in prefrontal synaptic transmission observed during the first day of extinction may reflect processing of the high degree of predictiveness of danger (unconditioned stimulus: US) by the aversive conditioned stimulus (CS). However, the subsequent potentiation of transmission in the mPFC may be related to processing of cognitive information such as the CS will no longer be followed by the US, even if emotional response (freezing) to the CS is still high.